Ultra wide-angle eyepieces employing positive power lens elements on one side of the internal focal plane and a negative power lens on the other side of the focal plane, i.e., between the positive lens and the telescope objective, have been disclosed heretofore as, for example, in my earlier U.S. Pat. No. 4,286,844. The eyepieces there disclosed were shown to be effective in achieving highly corrected flat fields of view. The improved field curvature is obtained by compensating the field curvature of the positive lens group by that of the negative lens. However, the use of the negative lens with strongly divergent rays requires a large aperture positive lens with attendant comparatively high manufacturing costs and weight. Consequently, the focal lengths of such wide angle eyepieces were kept relatively short, e.g., 13 mm., to minimize their weight.
Moreover, ultra wide-angle eyepieces, i.e., those having an apparent field of view exceeding 50 degrees, give rise to spherical aberration of the exit pupil. Rays with large exit angles will usually intersect the optical axis nearer to the last eyepiece lens element than those rays with small or moderate exit angle. This results in the so-called "kidney bean" effect when the circle of least confusion of the exit pupil of the eyepiece exceeds in size the pupil of the observer's eye. Thus, eyepieces as disclosed in my aforementioned patent were, for all practical purposes, limited to 13 mm. focal lengths for night observations and to 9 mm. focal lengths for daytime use.